This invention relates to a process for producing a polymer water-in-oil emulsion; and more particularly and in a preferred embodiment, to a process for producing a water-in-oil emulsion of a linear, high molecular weight polymer.
Water-in-oil emulsion polymerization processes, in which a water-soluble monomer is emulsified in an oil phase and polymerized therein, are well known in the art. For example, U.S. Pat. No. 3,284,393 describes such a process wherein water-soluble monomers are polymerized to high molecular weight polymers or copolymers utilizing a water-in-oil emulsion polymerization procedure. In the polymerization process described in said patent, one or a plurality of water-soluble monomers, or an aqueous solution thereof, are emulsified in an oil phase by means of a water-in-oil emulsifier and emulsion-polymerized under free radical forming conditions to form a polymeric latex in which the oil phase is the dispersion medium.
A water-in-oil emulsion of a polymer is produced by such a water-in-oil emulsion polymerization process, from which may be formed an aqueous solution of such polymer by inverting the emulsion with an inverting surfactant. Such "inverse emulsion polymerization" processes are an important part of the commercial production of certain types of water-soluble polymers where a liquid containing a high concentration of polymer is desired. For example, many anionic polymeric flocculants are high molecular weight, water-soluble polymers. The water-in-oil emulsion polymerization route to such polymers is the most significant, commercially-viable method that provides a liquid product containing a high loading (concentration) of such polymers. A liquid product is preferred in commercial flocculation for its ease in handling, transporting and rapid dissolution in water. Similar considerations hold for other types of high molecular weight, water-soluble polymers.
Due to a much higher dispersed phase/continuous phase ratio used in a water-in-oil emulsion than the more conventional oil-in-water emulsion and the requirement for good invertibility, the stability of a water-in-oil emulsion containing unreacted monomer is often only marginal. In fact, under the influence of a high shear field, in particular at elevated temperatures, a monomer emulsion can break down quite readily, resulting in phase separation. Polymerization of such an unstable monomer emulsion would inevitably lead to the formation of gels. Since the breaking down of a monomer emulsion by a shear field, which may result from a circulation pump, or a homogenizer, or in-line mixers, or a high speed of agitation, is more likely to occur at elevated temperatures, usually near or at the intended polymerization temperature, a stable monomer emulsion must be attained at this stage to prevent reactor fouling.
It has been noted that severe gelling problems (i.e., formation of coagulum) are encountered in attempting to form polymers in water-in-oil emulsions using a free-radical initiator. Since such polymerizations are highly exothermic, an external heat exchanger in a circulating loop used to cool the polymerization reaction often results in severe gellation of the entire mass. It was postulated that the high shear imparted to the emulsion, particularly at elevated temperature, during circulation in the external loop caused the breakdown of the emulsified droplets and their subsequent gellation.
This gellation problem was overcome by the novel use of two different kinds of initiators in the water-in-oil emulsion polymerization process. Specifically, it has been discovered that the shear stability of a monomer water-in-oil emulsion is drastically enhanced once a small amount of polymer is formed in situ in the monomer emulsion. In fact, such a monomer emulsion becomes shear-resistant even at elevated temperatures. A stable monomer water-in-oil emulsion can be achieved by initiating the polymerization during the heat-up process using a first, very reactive initiator and once a small amount of polymer is present therein, a stable water-in-oil emulsion results. The polymerization may then be completed by using a second, less reactive initiator at the desired reaction temperature. This process is described more fully in commonly-assigned, copending U.S. application Ser. No. 474,420, filed Mar. 11, 1983, the disclosure of which is hereby incorporated herein by reference.